Heretofore, the following method is known as a method for manufacturing a substrate of an electric device and the like. Specifically, a plurality of substrates are collectively built in one substrate. Dividing grooves are formed from front and back surfaces between unit substrates in the substrate. The substrate is divided into a plurality of unit substrates by the dividing grooves. Patent Literature 1 discloses an example of a dividing groove forming device used for such a substrate manufacturing method. This dividing groove forming device includes two cutters each including a plurality of rotary blades.